Process for preamble detection in a multi-stream 802.16E receiver

ABSTRACT

A preamble detector for a plurality of streams of baseband digitized signals has a plurality of preamble processors, each preamble processor coupled to an input and generating an output. Each preamble processor has an input coupled to a first delay, the output of the first delay coupled to a second delay generating an output. The first and second delay are substantially equal to a preamble part. A first multiplier generates an output from a conjugated output of the second delay output and a first delay output. A second multiplier generates an output from a conjugated first delay output and an input stream. The first and second multiplier outputs are accumulated over an interval, and the complex output of the accumulator is formed into a magnitude, thereby generating the output of each preamble processor. The outputs of the preamble processors are summed and compared to a threshold to generate a preamble detect.

FIELD OF THE INVENTION

The present invention relates to the detection of preambles transmittedby orthogonal frequency division multiple access (OFDMA) systems such asthose described in the IEEE 802.16e WiMAX standard. In particular, theinvention relates to the detection of preambles received by a SubscriberStation (SS) in the presence of interference from neighboring BaseStations (BS).

BACKGROUND OF THE INVENTION

The IEEE 802.16e-2005 standard uses Time Division Multiplexing (TDM) toenable multiple users to be serviced with data transfer, coordinated bya Base Station. Synchronized time slots are allocated for datatransmission from the BS to the Mobile Subscriber (MS), a direction ofdata flow known as a downlink, and for data transmission from the MS tothe BS, known as an uplink. Uplink communications and downlinkcommunications may use the same frequency band and are thereforedifferentiated by time slot. The 802.16e standard uses OrthogonalFrequency Division Multiplexing—Multiple Access (OFDMA) as themodulation method for transmitting data on the RF channel. These burstsof transmissions require the receiver to synchronize to the transmitterbefore data can be extracted from the signals. The Downlink sub-frametherefore starts with a Preamble and is followed by the data symbols.

FIG. 1 shows a preamble according to 802.16e standard where ‘N’ is theFFT size used in the downlink and ‘CP_LEN’ is the cyclic prefixassociated with any OFDM symbol. The possible values of ‘N’ are 512 and1024. Since N is not exactly divisible by three, the sample delay usedis round(N/3) where round( ) is a round-off function to the nearestinteger.

Each BS segment in a cell will transmit every 3rd sub-carrier in thepreamble according to the segment allocated to the BS as shown in FIG.2. It can be seen from FIG. 3 in combination with FIG. 2 that differentadjacent segments use different sets of subcarriers, and that theconfiguration of FIG. 2 is a typical cell arrangement. Onecharacteristic of the subcarriers as shown in FIG. 2 is that the Seg_0subcarriers will accumulate 0 degrees of phase shift over an N/3interval shown in FIG. 1, the Seg_1 subcarriers will accumulate 240degrees of phase shift over an N/3 interval, and the Seg_2 subcarrierswill accumulate 120 degrees of phase shift over an N/3 interval.

FIG. 4 shows the block diagram of a prior art wireless receiver. Antenna405 is coupled to RF front end 404 which converts the signal to basebandand has adjustable gain controlled by AGC 402, which is operative over apart of the preamble. ADC 406 samples the baseband converted signal toan IQ baseband data stream referred to as RX_IQ. The main function ofthe synchronization block 408 is to detect the frame boundary using thepre-defined preamble of the RX_IQ stream. The remote transmitter booststhe preamble in power by 3 dB compared to the data symbols which followthe preamble. The frame preamble is a repetitive sequence specificallydesigned for robust detection and identification in a receiver. Atypical implementation of preamble detection logic would use shiftedauto-correlation between the repetitions in the preamble, whereby thepreamble is correlated with time-shifted shifted version of itself witha shift of ‘N/3’ and the shifted correlation is accumulated over awindow having an extent of N/3+N/8.

FIG. 5 shows the computation of shifted auto correlation for an 802.16ereceiver, where the autocorrelator includes multiplier 510 which ismultiplying first values from the first preamble part 504 withconjugated 511 second values from second preamble part 506, and theresult of each multiply 510 is accumulated 512 over a window equal toN/8. The preamble can be detected by observing the output of theaccumulator 512 in combination with a threshold to make the preambledetection decision. The shifted correlation is a complex value with adefined plateau. The width of the plateau would be CP_LEN (N/8) due tothe accumulation of the correlation over a window of ‘N/8’.

The shifted correlation is computed as

${{shifted\_ corr}(k)} = {\sum\limits_{n = 0}^{{{CP}\;\_\;{LEN}} - 1}{{x\left( {k - n} \right)} \times {x^{*}\left( {k - n - D} \right)}}}$

Where CP_LEN is equal to ‘N/8’, and D is the delay representing theseparation between repeating preamble symbols used for the shiftedversion of the preamble.

FIG. 6 shows the property of shifted auto-correlation such as the output514 of accumulator 512. Magnitude 606 of the accumulator output iscompared to a threshold level 604 for detection of preamble, and phase602 shows a flat characteristic over the preamble correlation extent. Aswas described earlier, the phase plateau 603 for a Seg_0 segment will be0 degrees, the phase plateau 603 for a Seg_1 segment will be 240degrees, whereas the phase plateau 603 for a Seg_2 segment will be 120degrees. The particular threshold value for preamble detection is basedon the magnitude of the raw I and Q samples coming in, such that as theaverage signal energy increases, the threshold also increases so as tokeep noise or other random patterns from triggering a falsepreamble-detect event.

FIG. 7 shows the block diagram for prior art preamble detection withmultiple input receive streams, such as from multiple antennas as usedin multiple input multiple output (MIMO) wireless systems. Preambles 716and 718 represent the RX_IQ streams from a first and second ADCassociated with a first and second antenna, and the two streams areseparately conjugated (703, 707), multiplied (702, 706), and accumulated(704, 708). The first accumulator 704 and second accumulator 708 outputsare added 710 to form output 714, which is threshold compared, and theresultant value is used to determine the presence of a preamble.Multipliers 702 and 706 operate on complex values, and are used incombination with associated conjugators 703 and 707 to compute thedot-product of the complex preamble samples by multiplying them withdelayed and conjugated versions. The conjugation operation 703 and 707involves reversing the magnitude of the imaginary component. Theresultant dot product which is accumulated 704 and 708 is also complex.The accumulation is carried out over a predetermined window (N/3), whichwindowing can be done many ways, including subtracting the dot-productsof the N/3 delayed sample with a further N/3 delayed samples, therebycancelling out old accumulated values and preserving the desired currentwindow. Each accumulator is reset to zero when preamble detection isaccomplished so as to enable it to start afresh for a new preamble.During preamble search of the noise values which precede the preamble,the accumulator adds the shifted autocorrelation values of noise in theinter-frame gap, and due to the non deterministic or random nature ofnoise the accumulator does not hold any significant value during thatinterval.

The technique of FIG. 7 works well in the absence of interferers sincethe shifted correlation values from the two antenna paths add upconstructively to improve the correlation strength. FIG. 8 shows this inthe form of vectors, where a signal 802 from the RX_IQ stream associatedwith antenna 1 is added to signal 804 associated with RX_IQ signalstream of antenna 2 antenna to generate combined signal stream 806.

FIG. 9 shows the phase relationship between the segments of a basestation as was shown in FIG. 3. Each of Base stations C1 302, C2 304, C3306 transmit simultaneously on different segments S0, S1, S2 and atdifferent subcarrier combinations, as was shown in FIG. 2. Since each ofthe neighbor BS segments C1, C2, C3, etc transmit the preamble on everythird sub carrier as shown in FIG. 2, the resulting phases of shiftedcorrelation at the subscriber as shown in FIG. 9 will be 120 degreesapart. A parameter associated with a station is the “reuse number”,which indicates the number of frequencies deployed by the network. FIGS.2 and 3, in a reuse-1 scenario, show that the cell is divided intospatial zones serviced by different subcarriers in the preamble. Forreuse-1 interference the subscriber station might see the interferencein all three segments from neighboring Base Stations.

FIG. 9 shows the signal received from each interferer for reuse-3 whenthe subscriber receives the signal from all Base Stations with equalpower. As shown in the figure, if all the BS interferers are receivedwith the same power 902, 904, 906, the resulting signal will becancelled out and therefore the preamble cannot be detected. This is anexceptional scenario and is shown for example only. Naturally, thetypical signal from each base station BS1, BS2, BS3 will have adifferent level due to different multipath channels as shown in FIG. 10,where base stations BS1, BS2, BS3 sum to produce combined power 1116.The conventional combining of the Rx signal at the antennas will givebetter results in absence of BS Interferers but will prove ineffectivein presence of Interferers. FIGS. 11A and 11B show signal streamscombining at the first receive antenna A1 and second receive antenna A2in a typical multipath channel in presence of BS interferers. Theindividual antenna signals A1 1108 and A2 1116 of FIGS. 11A and 11B,respectively, combine as shown in FIG. 11C to produce a weaker signal1120 than either contributor 1108 or 1106 individually. It is then clearthat for certain signal conditions, the combined cross correlation ofthe individual RX_IQ streams combine in a destructive manner, therebydegrading the performance of the prior art preamble detection.

OBJECTS OF THE INVENTION

A first object of this invention is a preamble detector for OFDMAsignals which has, for each receive data stream, sample buffers, a firstand a second complex conjugator, a first and second multiplier, eachoperating on a sample buffer output and conjugator output, themultiplier outputs added and an absolute value formed, where each streamhas an associated absolute value output thus formed, the absolute valueoutputs summed to form an output to be threshold compared to detect apreamble.

A second object of the invention is a process for preamble detection,the process including, for each stream of incoming data, formingoriginal data, first delay data, and second delay data, multiplying thefirst delayed data with conjugated second delayed data to form a firstmultiplier output, multiplying the original data with conjugated seconddelay data to form a second multiplier output, summing and taking theabsolute value of the first multiplier output and second multiplieroutput, and summing each absolute value for each stream to compare witha threshold to form a preamble detect output.

SUMMARY OF THE INVENTION

A preamble detect includes, for each incoming signal stream, a firstdelay formed from an input signal, and a second delay formed from thefirst delay output. A first multiplier forms an output by multiplyingthe conjugated output of the second buffer with the output of the firstbuffer. A second multiplier forms an output by multiplying the inputsignal with the conjugate of the first delay output. The firstmultiplier and second multiplier outputs are summed and an absolutevalue is formed from the sum. The absolute values outputs associatedwith each incoming signal stream are summed to form a preamble detectionsignal, and the preamble detection signal may be compared with athreshold to form a preamble detection output.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of the preamble part of a wireless frame.

FIG. 2 shows the preamble subcarrier allocations for base stationsegments using reuse-3.

FIG. 3 shows how base station segments are structured in a cellularfashion.

FIG. 4 shows a block diagram of the baseband processing portion of anOFDM or OFDMA receiver.

FIG. 5 shows a block diagram for a shifted auto-correlator with a timediagram of a preamble showing samples used in the correlation.

FIG. 6 is a phase and magnitude plot of the shifted auto-correlationoutput.

FIG. 7 shows an arrangement for detecting preambles through multipleantennas.

FIG. 8 shows a vector diagram of signals combined from multiple antennasin the absence of neighboring interferers.

FIG. 9 shows a vector diagram of received preamble signals fromneighboring base stations having identical signal strength.

FIG. 10 shows a vector diagram of received preamble signals fromneighboring base stations with unequal strength.

FIGS. 11A and 11B show the combining of preamble detect signals fromneighboring stations at a first antenna and second antenna,respectively, for a base station.

FIG. 11C shows the autocorrelator response for a base station receivingsignals shown in FIGS. 11A and 11B.

FIGS. 12A and 12B show vector diagrams for power received at first andsecond antenna, respectively, for a base station which includesinterferers.

FIG. 12C shows a vector diagram for autocorrelation combining accordingto one embodiment of the invention.

FIG. 13 shows a block diagram and time diagram for preamble reception inone embodiment of the present invention.

FIG. 14 shows a signal processing block diagram according to the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

A receiver at the subscriber station of an OFDM or OFDMA based wirelessnetwork receives frames of data that are preceded by a specificallydesigned preamble sequence which has the property of repetition anddelay. The receiver is part of a wireless network which consists ofseveral base Stations (BS) that act as the control units and the conduitto any associated wired networks. The base stations are arranged in acellular fashion, and each base station divides its signals into threesectors each serviced with a different preamble operating on a differentsubcarrier. Each base station segment transmits its preamble in everythird subcarrier as allocated to it. At edges of the cells or atlocations where neighboring base stations are also visible to asubscriber station, the subscriber station is subjected to preamblesfrom the neighboring base stations, such signals resulting inpotentially destructive interference at the subscriber station (SS). Theability of the subscriber station to detect preambles of its basestation robustly is preserved even under such circumstances by using themagnitude of the preamble correlation seen at each of two antennas atthe subscriber stations so that the preamble correlations fromneighboring base stations are used to strengthen the preamble detectprocess.

In one embodiment of the present invention, the shifted correlationvalues from each antenna are combined after computing the magnitude ofthe correlation. Therefore the correlation values from interfering BaseStations combine constructively and improve the shifted correlationquality enabling detection of the preamble in presence of StrongInterferers as shown in FIG. 12.

FIG. 13 shows an overview diagram for the functional blocks in thepreamble detector of the present invention. Preamble 1302 is part of afirst RX_IQ baseband digitized stream which was received,down-converted, and digitized from a first antenna, and preamble 1326 ispart of a second RX_IQ baseband digitized stream which is similarlyprocessed from a second antenna. Each stream has N/8 samples of thecircular prefix (CP) and followed by a first preamble part 1304, asecond preamble part 1306, and a third preamble part 1308. The inputs tofirst multiplier 1310 encounter a differential delay equal to N/3 each,such that the first multiplier accepts an input associated with thestart of second preamble part 1306, while the other input of themultiplier accepts a delayed first preamble part 1304. A delaysubstantially equal to first preamble interval N/3 provides an inputwith a time relationship to the start of the first preamble 1304 tofirst multiplier 1310 with the other input an undelayed input of themultiplier associated with the start of second preamble 1306. A dotproduct (conjugation of one input and multiplication with the other) iscarried out on the samples at the beginning of the first N/3 block andthe beginning of the second N/3 block. The dot product is a complexmultiplication (1310, 1314) of the first operand with the complexconjugate (1311, 1313) of the second operand. Since a valid preamblealone would have the regular structure of repetitive N/3 samples, therewould be a strong correlation and therefore strong accumulation over aninterval of N/3. At the end of the preamble the correlation begins totaper down as shown in FIG. 6. The accumulator adds up the correlationvalues continuously. This is however reset to zero upon detection of theframe preamble. The accumulator is followed by a block that computes themagnitude of the result, where the magnitude output of each accumulator1312 and 1316 is computed by taking the square root of the positive realand imaginary parts squared. The first RX_IQ stream 1302 and secondRX_IQ stream 1326 are identically handled. The outputs of the magnitudefunctions 1313 and 1317 are added, resulting in a strongly positivecorrelation value which is immune from the destructive componentaddition shown in FIG. 11C of the prior art, and instead produces thevector addition shown in FIG. 12C.

FIG. 14 shows one embodiment of the invention for a two RX_IQ streamcase. Each stream 1401 and 1417 is separately handled by identicalpreamble processors 1403 and 1419, such that the preamble detector 1400may be expanded for any number of streams by the addition of anadditional preamble processor for each new stream. First stream 1401 isapplied to first preamble processor 1403 and is subject to a first delay1402 such as a N/3 sample buffer with a delay equal to a preamble part.The output of the first delay 1402 is applied to second delay 1404 suchas an N/3 sample buffer with a delay equal to a preamble part. Eachsample buffer 1402 and 1404 stores both I and Q values of the samplestream. The conjugate blocks 1406 and 1410 reverse the polarity of theapplied Q sample. The first multiplier block 1406 is a complexmultiplier operating on the conjugated output of the second delay andthe output of the first delay, carrying out 4 real multiplications andtwo additions per operation. The accumulator 1414 operates by adding therecently correlated values and subtracting the previous correlationvalues of the N/3 delayed path from 1404, although the accumulator canalso be realized by summing a fixed number of previous samples, or anyother prior art method for accumulation of samples over a fixed samplewindow size. The summer 1414 providing the N/3 accumulated output isprovided to the MAG block 1416 that performs sqrt(Re(.)^2+Im(.)^2),which output produces a magnitude value that is always positive, sincethe squared real and squared imaginary terms are positive values. Thefunctions of second preamble processor 1419 operate similarly, where theoutput is the magnitude 1432 of the accumulated difference 1430 wherebya number such as N/3 samples of the output of a first multiplier 1426 issubtracted from the output of second multiplier 1428. The firstmultiplier 1426 forms a complex product from the conjugate 1422 of theoutput of a second delay 1420 whose input is coupled to the output of afirst delay 1418. The second multiplier 1428 forms a product from anRX_IQ stream input 1418 coupled to the first delay 1418 and the outputof the first delay 1418 which is conjugated 1424.

The outputs of all of the preamble processors 1401 and 1417 are summed1434 together, and compared 1440 with a threshold 1436 to form apreamble detect output 1438. The threshold 1436 is based on themagnitude of the raw I, Q signals from the input RX_IQ streams 1401 and1417.

What is claimed is:
 1. A process for preamble detection in a pluralityof baseband receive data streams having at least one preamble of lengthN, the process having: a first step of delaying said receive data streamby an amount substantially equal to N/3, thereby producing a first delayoutput; a second step of delaying said first delay output by an amountsubstantially equal to N/3, thereby producing a second delay output; athird step of multiplying each conjugated second delay output by saidfirst delay output to generate a first multiplier output; a fourth stepof multiplying each conjugated first delay output by said receive datastream to generate a second multiplier output; a fifth step ofaccumulating said first multiplier output by subtracting said secondmultiplier output from said first multiplier output, thereafter forminga magnitude output from said accumulator output; a sixth step of summingall of the magnitude outputs and comparing the sum with a firstthreshold, asserting a preamble detect when said sum exceeds said firstthreshold.
 2. The process of claim 1 where said first delay and saidsecond delay correspond to the duration of a preamble part.
 3. Theprocess of claim 1 where said conjugated output changes the sign on animaginary part of a value.
 4. The process of claim 1 where said firstand said second multiplier are complex multipliers.
 5. The process ofclaim 1 where said accumulator operates over a number of samplessubstantially equal to the number of samples in a preamble part.
 6. Theprocess of claim 1 where at least one of said input streams iscompatible with IEEE 802.16e.
 7. The process of claim 1 where saidnumber of streams is two or three.
 8. The process of claim 1 where saidfirst step delay or said second step delay is a memory buffer having alength equal to N/3 samples rounded to the next integer.
 9. The processof claim 1 where said baseband data stream is derived from a wirelesssignal which is compatible with IEEE 802.16e.
 10. The process of claim 1where said first step delay or said second step delay utilizes a samplebuffer or FIFO to delay a particular data stream.
 11. A process forpreamble detection in a plurality of baseband receive data streamshaving at least one preamble of length N, the process having: a streammagnitude generation for each stream, the magnitude generation having: afirst step of delaying a particular receive data stream by an amountsubstantially equal to N/3, thereby producing a first delay output; asecond step of delaying said first delay output by an amountsubstantially equal to N/3, thereby producing a second delay output; athird step of multiplying each conjugated second delay output by saidfirst delay output to generate a first multiplier output; a fourth stepof multiplying each conjugated first delay output by said receive datastream to generate a second multiplier output; a fifth step ofaccumulating said first multiplier output by subtracting said secondmultiplier output from said first multiplier output, thereafter forminga magnitude output from said accumulator output; a preamble detectionstep whereby the magnitude output associated with each said stream issummed together into an accumulator and compared with a threshold value,and when said accumulator exceeds said threshold value, generating apreamble detect output.
 12. The process of claim 11 where said firstdelay or said second delay correspond to the duration of a preamblepart.
 13. The process of claim 11 where said conjugated output changesthe sign of the imaginary component of a value having a real andimaginary component.
 14. The process of claim 11 where at least one ofsaid first multiplier or said second multiplier are complex multipliers.15. The process of claim 11 where said accumulator operates over anumber of samples substantially equal to the number of samples in apreamble part.
 16. The process of claim 11 where at least one of saidinput streams is compatible with IEEE 802.16e.
 17. The process of claim11 where at least one said baseband receive data stream comprises anin-phase part and a quadrature part.
 18. The process of claim 11 wheresaid first step delay or said second step delay of N/3 is a buffer orFIFO having an integer number of samples closest to N/3.
 19. The processof claim 11 where said baseband data stream is derived from a wirelesssignal which is compatible with IEEE 802.16e.